Phonon Wave Research Articles

Phonon quarters-wave loss

There has been a growing interest in the phase of phonon, due to the theoretical prediction (Phys. Rev. Lett. 115.11 (2015)) and the experimental observation (Science 359.6379 (2018)) of chiral phonons, which have different phases in different components. While half-wave loss is a well-known concept in optics, in this work, a series of plateaus of quarters-wave loss are first found for the reflected phonon across an interface by using an atomic junction model. These plateaus can be understood by the S-matrix in the system with time-reversal symmetry. If a phonon wave propagates from a low acoustic-impedance material (or a low cutoff frequency material) to a higher one in the long-wave limit (or in the high frequency limit), a half-wave loss takes place for the reflected phonon; however, the plateau of half-wave loss for reflected phonon occurs in the whole frequency domain if phonon transfers to a material with a larger spring constant. Besides the half-wave loss, we also observe plateaus of quarter-wave (three-quarters-wave) loss in long wave limit when the two leads with identical acoustic impedance are coupled by a weak (strong) coupling in comparison with the optimum thermal coupling. The quarters-wave loss for phonons can be applied to chiral phonon manipulation and other phononics devices.

Moiré phonons in twisted bilayer graphene

We study the in-plane acoustic phonons in twisted bilayer graphenes using the effective continuum approach. We calculate the phonon modes by solving the continuum equation of motion for infinitesimal vibration around the static relaxed state with triangular domain structure. We find that the moir\'{e} interlayer potential only affects the in-plane asymmetric modes, where the original linear dispersion is broken down into miniphonon bands separated by gaps, while the in-plane symmetric modes with their linear dispersion are hardly affected. The phonon wave functions of asymmetric modes are regarded as collective vibrations of the domain-wall network, and the low-energy phonon band structure can be qualitatively described by an effective moir\'{e}-scale lattice model.

Periodicity of probability density and effects of Rashba Spin–orbit interaction on excitation energy of bound polaron in quantum pseudodot

Abstract We study the probability density and the excitation energy of the bound polaron in the quantum pseudodot under the Rashba spin–orbit interaction by using the unitary transformation of Lee–Low–Pines and the variational method of Pekar. The results of a numerical calculation show that the probability density of the bound polaron in the superposition state is periodic and oscillates with the period T 0 . Due to the Rashba spin–orbit interaction, the excitation energy splits into different branches. It also can be seen that the excitation energy changes with the phonon wave vector when the two-dimensional electron gas’s chemical potential, the electron–LO phonon coupling, the Coulomb bound potential and the Rashba coupling parameter take different values.

A phonon wave packet study of thermal energy transport across functionalized hard-soft interfaces

Thermal transport across hard-soft interfaces is critical to many applications, such as polymer composites, solar thermal evaporation, and photothermal cancer therapy. In this work, we use wave packets (WP) in molecular dynamics (MD) simulations to study the phonon energy transmission coefficients (ETCs) across different Au-self-assembled monolayer (SAM)-organic liquid interfaces. Three types of thiol SAMs with different terminal groups and chain length heterogeneities are studied, including “-CH3,” “-COOH,” and “hetero” SAMs. Two types of organic liquids, hexamine and hexane, are investigated. When the liquid changes from hexamine to hexane, the Au-CH3 SAM-liquid interfaces show similar ETCs across different phonon modes, since the interactions between nonpolar SAM and different liquids are similar, while the ETCs across the Au-CH3 SAM-liquid interfaces are much higher than those involving bare Au-liquid interfaces. Due to the -COOH functionalization, the Au-COOH SAM-hexamine interface shows the highest ETCs for all phonon modes compared to the other interfaces, which explains why its interfacial thermal conductance (ITC) is also the highest. We find that the Au-hetero-SAM-hexamine interface has higher ETCs in the longitudinal acoustic (LA) modes than the Au-CH3 SAM-hexamine interface; as a result, the ITC of the Au-hetero-SAM-hexamine interface is slightly higher. The ETCs calculated in our WP simulations can interpret the ITCs calculated from our previous MD simulations. Using the mode-resolved ETCs, the ITC contributions for each phonon mode are also calculated. We find that the LA modes play an important role in thermal transport across Au-SAM-liquid interfaces. Results from this WP study will help design interfaces with desirable thermal transport properties.

The electric field of an electron in an electron–hole plasma with degenerate electrons: Possibility of formation of a superconductivity state

We consider the possibility of the formation of a superconductivity state either in a semiconductor or in an electron–hole plasma with degenerate electrons due to the attractive forces between the electrons as a result of the exchange effects through the electron–hole sound wave by an analogy to the phonon waves in a solid state. We have determined an interaction potential between two electrons in a degenerate electron–hole plasma. The potential appears to be attractive at distances much larger than the Debye radius and decreases as 1/r3. We discuss the conditions in which the bound electron state, the so‐called “Cooper Pair,” in a such field can be formed.

Brillouin light scattering under one-dimensional confinement: Symmetry and interference self-canceling

We present the spontaneous Brillouin light scattering (BLS) under simultaneous one-dimensional confinement of sound and light and show that the photon-phonon coupling results from nontrivial interplay of the photoelastic and moving-interface effects. We reveal two types of BLS self-canceling: governed by mode symmetry and driven by destructive interference of the two effects. We show that the latter can be adjusted by the light polarization and phonon wave number. Furthermore, we present a measurement of the shear-horizontal waves in thin membranes.

Thermoelectric Enhancement of Silicon Membranes by Ultrathin Amorphous Films.

We propose a simple, low-cost, and large-area method to increase the thermoelectric figure of merit (ZT) in silicon membranes by the deposition of an ultrathin aluminum layer. Transmission electron microscopy showed that short deposition of aluminum on a silicon substrate covers the surface with an ultrathin amorphous film, which, according to recent theoretical works, efficiently destroys phonon wave packets. As a result, we measured 30-40% lower thermal conductivity in silicon membranes covered with aluminum films while the electrical conductivity was not affected. Thus, we have achieved 40-45% higher ZT values in membranes covered with aluminum films. To demonstrate a practical application, we applied this method to enhance the performance of a silicon membrane-based thermoelectric device and measured 42% higher power generation.

Anderson Localization Quenches Thermal Transport in Aperiodic Superlattices.

We show that aperiodic superlattices exhibit intriguing interplay between phononic coherent wave interference effects and incoherent transport. In particular, broadband Anderson localization results in a drastic thermal conductivity reduction of 98% at room temperature, providing an ultralow value of 1.3 W m^{-1} K^{-1}, and further yields an anomalously large thermal anisotropy ratio of ∼10^{2} in aperiodic Si/Ge superlattices. A maximum in the thermal conductivity emerges as an unambiguous consequence of phonon Anderson localization at a system length scale bridging the extended and localized transport regimes. The frequency-resolved picture, combined with our lattice dynamical description of Anderson localization, elucidates the rich transport characteristics in these systems and the potential of correlated disorder for sub- to few-THz phononic engineering of heat transport in thermoelectric applications.

Electrostatically Induced Phononic Crystal

The possibility of realizing an electrostatically induced phononic crystal is investigated numerically in an acoustic waveguide based on a graphene sheet that is suspended over periodically arrayed electrodes. The application of dc voltage to these electrodes exerts electrostatic force on the graphene and this results in the periodic formation of stress in the waveguide structure in a non-invasive way, unlike the cases with mass loading and air holes. This non-invasive scheme enables a bandgap, namely a phononic crystal, to be created in the waveguide that can be used to dynamically tune the acoustic transparency in the medium. Our approach will allow the dispersion relation to be locally modified, thus modulating the temporal response of traveling acoustic phonon waves. This novel phonon architecture is promising in terms of realizing the advanced control of phonon dynamics such as waveform and dissipation engineering in the device.

Quantifying phonon particle and wave transport in silicon nanophononic metamaterial with cross junction

Understanding phonon transport mechanisms in nanostructures is of great importance for delicately tailoring thermal properties. Combining phonon particle and wave effects through different strategies, previous studies have obtained ultra-low or super-high thermal conductivity in different nanostructures. However, phonon particle and wave effects are coupled together, and thus, their individual contributions to phonon transport have not been figured out. In this article, we present how to quantify the particle and wave effects on phonon transport by combining Monte Carlo and atomic Green's function methods. We apply it to one-dimensional silicon nanophononic metamaterial with cross junctions, where it has been thought that the wave effect was the main modulator to block phonon transport and the particle effect was negligibly weak. Surprisingly, we find that the particle effect is quite significant as well and can contribute as much as 39% to the total thermal conductivity reduction. Further phonon transmission analysis by reducing the junction leg length also qualitatively demonstrates the strong particle effect. The results highlight the importance of mutually controlling particle and wave characteristics.

Phononic band gap and wave propagation on multiferroic-based acoustic metamaterials

In the present work, the acoustic band structure of a two-dimensional (2D) phononic crystal containing a multiferroic and liquid were investigated by the plane-wave-expansion method. 2D PnC with triangular and honeycomb lattices composed of LiCu2O4 cylindrical rods embedded in the seawater matrix are studied to find the existence of stop bands for the waves of certain energy. Phononic band diagram ω=ω(k) for a 2D PC, in which nondimensional frequencies ωa/2πc (c-velocity of wave) were plotted versus the wavevector k along the г-X-M-г path in the Brillouin zone show few stop bands in the frequency range between 10 and 110 kHz.

Phonon transfer in silicon-diamond films: Influence of thermal boundary resistance on acoustic phonon intensities

Thermal energy transfer across two-dimensional silicon and diamond thin films are considered and the influence of the thermal boundary resistance, due to mismatching of properties, on the acoustic phonon transport characteristics is studied. The transient form of the Boltzmann equation with frequency dependency of phonons is incorporated simulating the phonon transport. Thermal boundary resistance is considered incorporating mismatches of interface conditions of films, namely cut-off and diffusive models. The ratios of longitudinal and transverse acoustic phonon wave intensities because mismatch models (cut-off and diffusive mismatches) are predicted for temperature oscillating boundary condition at the silicon edge. It is found that the ratio of the longitudinal acoustic phonon intensities ratio because of mismatch models (cut-off over diffusive) does not behave exactly the oscillation of temperature at the edge of the silicon film due to scattering of phonons in the films. The same arguments are applicable for the ratio of transverse acoustic phonon intensities because of the interfacial mismatch models (cut-off over diffusive). The consideration of the phonon frequencies cut-off results in the ballistic phonon transfer in between films, which lowers the interfacial thermal resistance.

Phonon localization in heat conduction.

Nondiffusive phonon thermal transport, extensively observed in nanostructures, has largely been attributed to classical size effects, ignoring the wave nature of phonons. We report localization behavior in phonon heat conduction due to multiple scattering and interference events of broadband phonons, by measuring the thermal conductivities of GaAs/AlAs superlattices with ErAs nanodots randomly distributed at the interfaces. With an increasing number of superlattice periods, the measured thermal conductivities near room temperature increased and eventually saturated, indicating a transition from ballistic to diffusive transport. In contrast, at cryogenic temperatures the thermal conductivities first increased but then decreased, signaling phonon wave localization, as supported by atomistic Greenșs function simulations. The discovery of phonon localization suggests a new path forward for engineering phonon thermal transport.

Anomalous K-Point Phonons in Noble Metal/Graphene Heterostructure Activated by Localized Surface Plasmon Resonance.

The metal/graphene interface has been one of the most important research topics with regard to charge screening, charge transfer, contact resistance, and solar cells. Chemical bond formation of metal and graphene can be deduced from the defect induced D-band and its second-order mode, 2D band, measured by Raman spectroscopy, as a simple and nondestructive method. However, a phonon mode located at ∼1350 cm-1, which is normally known as the defect-induced D-band, is intriguing for graphene deposited with noble metals (Ag, Au, and Cu). We observe anomalous K-point phonons in nonreactive noble metal/graphene heterostructures. The intensity ratio of the midfrequency mode at ∼1350 cm-1 over G-band (∼1590 cm-1) exhibits nonlinear but resonant behavior with the excitation laser wavelength, and more importantly, the phonon frequency-laser energy dispersion is ∼10-17 cm-1 eV-1, which is much less than the conventional range. These phonon modes of graphene at nonzero phonon wave vector (q ≠ 0) around K points are activated by localized surface plasmon resonance and not by the defects due to chemical bond formation of metal/graphene. This hypothesis is supported by density functional theory (DFT) calculations for noble metals and Cr along with the measured contact resistances.

Phononic wave hard limiter

In this paper nonlinear Schrödinger equation (NSE) is proposed to evaluate phononic wave propagation in nonlinear media regarding mass density as the fundamental parameter of nonlinearity. This equation is used to analyze the phononic wave limiting in one dimensional periodic structure. To approach this goal, considering the analogy, the mass density in elastic wave (EW) is related to the refractive index in electromagnetic (EM) wave and thus the nonlinear terms of the refractive index in EM correspond to nonlinear coefficient of the mass density in EW. Consequently, elastic wave coupled-mode (CM) equations are derived for backward and forward waves. Eventually, the CM equations are numerically solved to demonstrate the limiting process for phononic wave.

Characterization of longitudinal acoustic phonons in InGaAsP multiple quantum wells by asynchronous optical sampling

We performed time-resolved reflectivity measurements and characterized the time-varying oscillations of longitudinal acoustic phonons in InGaAsP multiple quantum wells (MQWs) by asynchronous optical sampling with Er-doped fiber lasers. Oscillations of longitudinal acoustic phonons ranging from a few tens to a few hundred gigahertz were observed, which were induced by forward and backward Raman scattering in Brillouin and first zone-folded acoustic phonon modes. To investigate the dynamics of each phonon mode for various pump-probe time delays, we analyzed the experimental data with a continuous wavelet transformation (CWT) and a subsequent modified inverse CWT. For high-frequency oscillations, we observed the oscillatory profile with beats and identified the contribution of each phonon mode. For low-frequency oscillations, a series of dissipations of phonon wave packets from each QW to the InP substrate was deduced from the analyzed data. These results suggest that our analysis method is useful for investigating the dynamics of acoustic phonons.

Longitudinal Optical Phonons Modified by Organic Molecular Cation Motions in Organic-Inorganic Hybrid Perovskites.

We performed terahertz time-domain spectroscopy for methylammonium (MA) lead halide perovskite single crystals and characterized the longitudinal optical (LO) phonons directly. We found that the effective LO phonon wave number does not change in the wide temperature range between 10 and 300K. However, the coupling between MA cation modes and the LO phonon mode derived from lead halide cages induces a mode splitting at low temperatures and a damping of the LO phonon mode at high temperatures. These results influence the interpretation of electron-LO phonon interactions in perovskite semiconductors, as well as the interpretations of mobility, carrier diffusion, and polaron formation.

Kohn anomaly of optical zone boundary phonons in uniaxial strained graphene: Role of the Dirac cone electronic dispersion

One of the unique properties of graphene is its extremely high mechanical strength. Several studies have shown that the mechanical failure of graphene sheet under a tensile strain is due to the enhancement of the Kohn anomaly of the zone boundary transverse optical phonon modes. In this work, we derive an analytical expression of the Kohn anomaly parameter $\alpha_{\vec{K}}$ of these phonons in graphene deformed by a uniaxial strain along the armchair direction. We show that, the tilt of Dirac cones, induced by the strain, contributes to the enhancement of the Kohn anomaly under a tensile deformation and gives rise to a dominant contribution of the so-called {\it outer} intervalley mediated phonon processes. Moreover, the Kohn anomaly is found to be anisotropic with respect to the phonon wave vectors around the K point. This anisotropy may be at the origin of the light polarization dependence of the Raman 2D band of the strained graphene. Our results uncover, not only, the role of the Kohn anomaly in the anisotropic mechanical failure of the graphene sheet, under strains applied along the armchair and zigzag directions, but shed also light on the doping induced strengthening of strained graphene.

Synthesis, structure, UV–Vis–NIR, infrared and Raman spectroscopy, and force-field investigation for A2GaSbO7 (A3+ = Y, Dy, Gd) pyrochlores

Three mixed pyrochlores, A2GaSbO7 (A3+ = Y, Dy, Gd) were synthesized using the solid-state reaction technique and characterized systematically through powder X-ray diffraction and its Rietveld analysis, SEM-EDAX, UV–Visible absorption spectra, Fourier-transform infrared and Raman spectroscopy. Scherrer analysis of the XRD pattern and SEM-image were used to estimate size of the nano-structured particles of the polycrystalline samples to be ∼40–50 nm. From the UV–Vis absorbance data, absorption band edge and optical band gap were determined, and found to be ∼350 ± 8 nm and 2–2.5 eV, respectively, for these compounds. Four F1u bands were only observed in the infrared spectra within the phonon wave numbers range of 400–800 cm−1 for these samples. The highest wave number infrared mode arises due to the interaction between bending and stretching of A−O and B−O bonds. A mode-splitting caused by the mass-difference of B-site ions was found in the other three infrared modes. The Raman spectra of these compounds exhibit six Raman modes, e.g., A1g, Eg, and 4F2g, within the experimental range of 50–1000 cm−1, with few extra bands in the higher (>700 cm−1) and lower (<200 cm−1) wave numbers range. A normal coordinate analysis using a generalized valence force-field model has been performed to assign and evaluate the vibration modes in these compounds. Five stretching and four bending force constants were considered to construct the force-field matrix. Comparison of the calculated phonon wave numbers with experimental values obtained in infrared and Raman excitation zones describes excellent agreement for all these isomorphous compounds.

Raman and time-resolved pulsed photoacoustic spectroscopy of solid trinitrotoluene in graphite mixture: For identification of double resonant optical phonon signatures

The optical phonon waves transfer mechanism between pure graphite (GP) and trinitrotoluene (TNT) using 1064 nm as an excitation wavelength based on the double resonant Raman effect and the pulsed photoacoustic (PA) technique has been investigated. The study was carried out at a different weight percentage of GP within TNT with respect to the two excitation wavelengths of 532 nm and 1064 nm (30 ps, 10 Hz, Q-switched Nd: YAG laser). The equal and double proportional matrix of the GP and TNT (solid samples) generate a strong PA signal with high quality factor (Q-factor) due to the interaction of optical phonons of GP and TNT in the same phase. The Raman spectra, recorded at 785 nm wavelength provides an insight on the pi-electron coupling between the GP and TNT. The obtained experimental results confirm that the pure GP and its derivatives, which have a SP2/SP3 hybridization, can be used to develop a new sensing optode for the detection and identification of explosive molecules in the visible and near-IR wavelengths range.